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Our role in the global accessibility movement is to empower all developers to build technology without barriers. First and foremost among those developers is developers with disabilities. We believe every single person should be able to build and create. As a blind developer myself, I know that one of our best points of leverage for improving accessibility is to include people with disabilities as equal participants within development teams.

Now, as Microsoft looks toward leveraging D3D12 for more layering solutions similar to OpenGLOn12 and OpenCLOn12, the legacy Resource Barrier model is becoming even more burdensome as compatibility issues arise.

A Resource State encapsulates both the Layout of a subresource and the ways the GPU can access a subresource (e.g. UAV write, SRV read, Render Target, etc). Resource State Transitions do the following:

App developers must assume that a State Transition Barrier will flush all preceding GPU work potentially-using StateBefore, and block all subsequent GPU work potentially-using StateAfter until the barrier is completed. However, this is often performed using naive, worst-case synchronization, resulting in longer-than-necessary latency.

For example, a transition from D3D12_RESOURCE_STATE_UNORDERED_ACCESS to D3D12_RESOURCE_STATE_NON_PIXEL_SHADER_RESOURCE|D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE will wait for ALL preceding Graphics and Compute shader execution to complete, and block ALL subsequent Graphics and Compute shader execution. This could be a problem if the preceding access was only in a Compute shader and subsequent access is only in a pixel shader.

D3D12 requires a full finish and flush of all GPU work at ExecuteCommandLists completion. Therefore, any writes to a resource during a preceding ExecuteCommandLists scope will be fully completed when that resource is needed in a subsequent ExecuteCommandLists scope. However, drivers are not expected to keep track of when a resource was last written to. So in addition to sync, a transition from a WRITE state may force a cache flush, even if the resource is already up-to-date from an earlier cache flush.

Assume a texture was used as a render target during a previous frame. Now the app wants to read from that texture in a pixel shader. Obviously a layout change may be required, but there is no need to flush any preceding Draw calls in the current ExecuteCommandLists scope. Unfortunately, a subresource state transition from D3D12_RESOURCE_STATE_RENDER_TARGET to D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE will unnecessarily force all preceding Draw commands to finish and flush before transitioning the texture layout.

Under the covers, an Aliasing Barrier typically blocks all GPU execution until all preceding work is finished and writes are flushed. This is quite expensive, especially if resource aliasing is being used to improve application efficiency.

Implicit State Transitions (promotion and decay) was invented to help support such scenarios. Unfortunately, Implicit state promotion and decay is a major source of confusion for developers. There are complex - and evolving - rules about when promotion and decay occur.

Promotion and decay reflect the natural consequences of ExecuteCommandLists boundaries. However, some developers incorrectly assume that hidden barriers are being inserted behind the scenes. As such, it is common for promotion and decay to be ignored, resulting in excessive use of unnecessary barriers with noticeable performance impact.

The D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE state is only usable in Direct command lists. Therefore, a Compute queue cannot use or transition a resource in state D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE|D3D12_RESOURCE_STATE_NON_PIXEL_SHADER_RESOURCE. However, Compute queues DO support D3D12_RESOURCE_STATE_NON_PIXEL_SHADER_RESOURCE, which has an identical layout in both Direct and Compute queues. This design oversight is a common source of d3d12 app developer frustration. The primary reason for the separate states is to provide precise execution sync and memory flush for a Direct queue. However, when passing resources between Direct and Compute queues, sync and flush are handled using fences.

According to earlier D3D12 specifications, Clear, Copy or Discard commands require destination resources be in a specific resource state. Typically, this involves a state transition from a prior state. This makes sense when only a portion of the resource is being written to or discarded. However, when performing a full-subresource Clear, Copy or Discard, the old resource data is being completely replaced. Therefore, a layout transition seems unnecessary.

Buffers and simultaneous-access textures can be written to in one queue while concurrently being read-from one or more OTHER queues. However, this pattern is not supported in a single queue because legacy Resource Barrier design prevents subresources from being in both READ and WRITE states at the same time. However, hardware can support same-queue simultaneous-access; Layout is immutable and there is no sync or flush needed since the reads are non-dependent.

Resource State Transition Barriers provide a choice of transitioning either ALL subresources (subresource index set to 0xFFFFFFFF) or a single subresource. This may be inefficient when transitioning a large range of logically-adjacent subresources, such as a range of array slices or a full mip-level chain in a single array slice. It can be costly to build the array of individual transition elements and the translation from [mip-level, array-slice, plane-slice] to subresource-index is a common source of app bugs.

According to the D3D12 specifications, a subresource cannot be in a state that combines read-only bits and write bits. Therefore a resource cannot be in the D3D12_RESOURCE_STATE_COPY_SOURCE|D3D12_RESOURCE_STATE_COPY_DEST state. This rule applies for promoted states as well, so a resource in a COMMON state cannot be implicitly promoted to both COPY_SOURCE and COPY_DEST at the same time.

Texture Barriers control cache flush, memory layout and synchronization for texture subresources. Texture Barriers must only be used with texture resources. Texture barriers allow a selection of a single subresource, all subresources, or a coherent range of subresources (i.e. mip range and array range). Texture barriers must provide a valid, non-NULL resource pointer.

Buffer Barriers control cache flush and synchronization for buffer resources. Buffer Barriers must only be used with buffer resources. Unlike textures, buffers have only a single subresource and do not have a transitionable layout. Buffer barriers must provide a valid, non-NULL resource pointer.

Global barriers control cache flush and synchronization for all indicated resource access types in a single command queue. Global Barriers have no effect on texture layout. Global Barriers are needed to provide functionality similar to legacy NULL UAV barriers and NULL/NULL aliasing barriers.

Since global barriers do not transition texture layout, global barriers may not be used in transitions that otherwise would require a layout change. For example, a global barrier cannot be used to transition a non-simultaneous-access texture from D3D12_BARRIER_ACCESS_RENDER_TARGET to D3D12_BARRIER_ACCESS_SHADER_RESOURCE, since that would also require a change from D3D12_BARRIER_LAYOUT_RENDER_TARGET to D3D12_BARRIER_LAYOUT_SHADER_RESOURCE.

With legacy Resource Barriers, drivers must infer which work to synchronize. Often this is a best-guess since the driver may not be able to determine when a subresource was last accessed. Typically, the driver must assume the worst-case: any previous work that could have accessed a resource in StateBefore must be synchronized with any work that could access the resource in StateAfter.

D3D12_BARRIER_SYNC_NONE indicates synchronization is not needed either before or after barrier. A D3D12_BARRIER_SYNC_NONE SyncBefore value implies that the corresponding subresources are not accessed before the barrier in the same ExecuteCommandLists scope. Likewise, a D3D12_BARRIER_SYNC_NONE SyncAfter value implies that the corresponding subresources are not accessed after the barrier in the same ExecuteCommandLists scope. Therefore, Sync[Before|After]=D3D12_BARRIER_SYNC_NONE must be paired with Access[Before|After]=D3D12_BARRIER_ACCESS_NO_ACCESS.

If a barrier SyncBefore is D3D12_BARRIER_SYNC_NONE, then AccessBefore MUST be D3D12_BARRIER_ACCESS_NO_ACCESS. In this case, there MUST have been no preceding barriers or accesses made to that resource in the same ExecuteCommandLists scope.

If a barrier SyncAfter is D3D12_BARRIER_SYNC_NONE, then AccessAfter MUST be D3D12_BARRIER_ACCESS_NO_ACCESS. Afterward, there MUST be no subsequent barriers or accesses made to the associated resource in the same ExecuteCommandLists scope.

ClearUnorderedAccessView* operations are treated as UAV accesses and have their own synchronization scope, D3D12_BARRIER_SYNC_CLEAR_UNORDERED_ACCESS_VIEW. The ClearRenderTargetView and ClearDepthStencilView operations use D3D12_BARRIER_SYNC_RENDER_TARGET and D3D12_BARRIER_SYNC_DEPTH_STENCIL, which are shared with RT and DSV access scopes during Draw.

Execute barrier without waiting for preceding work or blocking subsequent work. This is something an app might do in a ExecuteCommandLists call that only performs Barriers to JIT (Just-In-Time) latch resource state. When working with enhanced barriers, this should only be used for transitioning texture subresources to a new layout. Given that completion of ExecuteCommandLists guarantees full pipeline sync and cache flush, there is zero value in using other barrier types in this way. 152ee80cbc

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